Coal pulverizers are extensively used in the power-generating industry to process coal into finely ground "fines" suitable for combustion. A common type of pulverizer is the bowl mill pulverizer, in which a bowl- or ring-shaped grinding plate is rotated while heavy grinding wheels crush and grind coal fed onto the plate from a feedpipe. Typically, a circular "throat" surrounds the outer edge of the grinding plate, and a stream of forced air is blown upward around the grinding plate to entrain the ground coal into a flow which spirals up and around the pulverizer into a classifier cone. Once in the classifier cone, the coal/air flow should be directed to swirl down into the classifier cone with a centrifugal classifying action, with the smaller coal fines separated up and out for combustion, and with larger coal particles not suitable for combustion swirled around the sides of the cone to eventually drop back into the pulverizer for regrinding.
The classifier cones are typically provided at their upper end with a circular classifier cage defining a circumferential inlet for the cone, the cage being filled with a number of classifier vanes which are used to direct the coal/air flow into the cone in a desired swirl pattern. Control over this swirl pattern is critical in maximizing control of the cone's classifying action, and the resulting fineness of the coal being burned.
The initial rotational direction of the coal/air flow around the pulverizer is imparted by a number of angled throat vanes in the throat, and subsequently modified by the classifier vanes to flow down and around inside the classifier cone. In the past, pulverizer throats have typically been stationary. Recently, however, the industry has been converting from stationary to rotating throats to improve flow efficiency from the throat. U.S. Pat. No. 4,721,258 to Dougan et al. describes a number of reasons for conversion from stationary to rotating throats. The Dougan et al. patent discloses an arrangement of pulverizer throat vanes (FIG. 4) in which the rotating throat vanes are oriented in the direction of rotation of the bowl and throat. This orientation is intended to take advantage of a specially-shaped throat vane having an airfoil portion.
However, it has since been found that orienting the throat vanes opposite the direction of the bowl/throat rotation is far more efficient, and has generally become the industry standard for rotating throat pulverizers. This results in a corresponding reversed rotation of the flow that reaches the classifier cage from the pulverizer throat.
Adjustment of the flow through the classifier cage is achieved with one of two types of adjustable vanes: fixed pitch vanes with lengthwise adjustable slide plates, and pivot-type vanes. The slide- and pivot-adjustments are intended to improve control over the flow into the classifier cone.
Prior art classifier vanes with lengthwise adjustments have been found not to help fineness control since they do not adjust tangential flow direction with respect to the interior surface of the cone. The pivot-type vanes offer better control over flow direction, using individual pivot adjustments or linkages to articulate sets of multiple vanes at the same time. However, pivoting alone is not sufficient to optimize directional control over the flow. Accordingly, some prior art vanes are additionally curved to help redirect flow.